PL/Perl: Fix compiler warning

The code was assigning a (Datum) 0 to a void pointer.  That creates a
warning from clang 3.4.  It was probably a thinko to begin with.

src/pl/plperl/plperl.c

@@ -1651,7 +1651,7 @@ plperl_inline_handler(PG_FUNCTION_ARGS)
 	/* Set up a callback for error reporting */
 	pl_error_context.callback = plperl_inline_callback;
 	pl_error_context.previous = error_context_stack;
-	pl_error_context.arg = (Datum) 0;
+	pl_error_context.arg = NULL;
 	error_context_stack = &pl_error_context;
 
 	/*